The present invention relates generally to AM-FM modulation and demodulation systems and specifically communication system with a gated Amplitude-Frequency (AM/FM) transmitter and an AM/FM receiver.
Due to the many growing uses of radio communication channels, there exists a shortage of channels in the radio frequency spectrum. An alternative to using multiple radio frequency carriers is the communication system of the present invention, which can transmit and receive, either alternatively or simultaneously, the same or different signals on one radio resulting in the availability of two separate channels of information (one AM and one FM) on the same carrier.
The task of providing a communication system capable of using two separate channels of information (one AM and one FM) on the same carrier has been alleviated, to some degree, by the prior art devices of the following U.S. patents:
U.S. Pat. No. 3,378,773 issued to Jeffers; PA0 U.S. Pat. No. 4,310,920 issued to Hayes; PA0 U.S. Pat. No. 4,001,702 issued to Kaufman; PA0 U.S. Pat. No. 4,070,628 issued to Funabashi; PA0 U.S. Pat. No. 4,304,004 issued to von der Neyen; PA0 U.S. Pat. No. 3,206,680 issued to Mason; PA0 U.S. Pat. No. 3,665,507 issued to Peil; PA0 U.S. Pat. No. 3,745,467 issued to Lundquist et al; PA0 U.S. Pat. No. 3,919,645 issued to Ohsawa et al; and PA0 U.S. Pat. No. 3,971,988 issued to Denenberg.
Radio receivers possessing both AM and FM reception are very common, especially in the automotive field. The Kaufman, Funabashi, von der Neyen, Mason, Peil, Lundquist et al, Ohsawa et al and Denenberg patents, which are incorporated herein by reference are all radio receivers capable of receiving AM and FM. However, most automotive radio receivers, such as the one of Funabashi, Mason, Peil, Lundquist et al, Ohsawa et al, and Denenberg receive AM and FM signals as alternatives; that is, they allow the user to select between reception of AM and FM signals.
What is desired is a communication system which allows simultaneous reception and simultaneous use of AM and FM channels on the same RF carrier.
The Kaufman disclosure does, in fact, simultaneously demodulate both the AM and FM signals. However, in this patent, the demodulated AM signal is used to squelch the FM demodulated data channel when noise or interference reaches a predetermined level. Similarly, von der Neyen's apparatus for transmission and reception of frequency modulated digital communication signals includes a blockable phase follow-up device for compensating for large differences between AM and FM demodulation products.
Both Kaufman and von der Neyen recognize that, theoretically, received FM signals have no AM components, so if there is no noise and if there is no interference, the output of the AM part of the demodulator is constant DC voltage. However, all the prior art radio receivers recognize the importance of avoiding interference between AM and FM signals, and the devices of Kaufman and von der Neyen are particularly tailored to eliminate such interference.
The radio receiver of the present invention differs from the prior art devices in both structure and intent. The present receiver demodulates AM and FM simultaneously to provide two separate channels of information (one AM and one FM) for simultaneous use from the same RF carrier.
Radio transmitters capable of transmitting both AM and FM are rare compared to the number of receivers patented. The Jeffers and Hayes patents incorporated herein by reference, are for radio transmitters capable of transmitting both AM and FM.
The transmitter of the present invention differs in structure and method of signal processing from the prior art devices of both Jeffers and Hayes. The transmitter of the present invention simultaneously amplitude modulates one input signal and frequency modulates a second input signal by first accomplishing frequency modulation on the first input signal. The carrier is then amplified, and the amplitude modulated by either the same, or a second input signal and transmitted.
In view of the foregoing discussion it is apparent that there currently exists the need for a communication system with the capability of transmission and reception of two separate channels of information (one AM and one FM) for simultaneous use on the same RF carrier. The present invention is directed towards satisfying that need.